Prior art AC plasma display panels (PDPs) generate the majority of their emitted light from the negative glow region of a gas discharge. As is known to those skilled in the art, gas discharges exhibit two distinct light emitting regions, i.e. the negative glow wherein a plasma exists with an excess of positively charged ions and the positive column wherein the plasma evidences a balance of positively charged ions and electrons.
PDP subpixel sites operate using the same fundamental principle as a fluorescent lamp. More particularly, a PDP subpixel employs ultraviolet light generated by a gas discharge to excite visible light emitting phosphors. A fluorescent lamp uses the positive column region of a gas discharge to generate most of its light since the positive column has a much higher luminous efficiency than the negative glow.
The positive column has not been previously successfully applied to AC PDPs because the limited physical space of the small subpixel sites do not easily allow sufficient room for the usually long dimensions of a positive column.
The Positive Column and the Negative Glow
In qualitative terms, the power in a gas discharge is divided between the two major regions: the positive column and the negative glow. The positive column is characterized by an equal density of electrons and ions that are of a very high density that shields out most of an applied electric field. In the positive column the high density of highly conductive electrons and ions quickly move to cancel any high field region.
The negative glow is characterized by a very high level of positive ions and a very low level of negative electrons. The high density of positive charge means that the electric field in the negative glow is very high. The very high electric field allows a major part of the potential applied across the gas to be dropped across the negative glow. Since the positive column and the negative glow are electrically "connected" in series, all of the current through the gas discharge passes through both the positive column and the negative glow. To determine the instantaneous power dissipated in a given discharge region, it is necessary to simply multiply the discharge current by the voltage drop across the region.
The positive column and the negative glow have considerably different luminous efficiencies. In general, the positive column is very efficient and the negative glow is inefficient. A fundamental reason for this difference is that most of the current flow in the positive column is due to electrons and most of the current flow in the negative glow is due to ions. Energy absorbed by electrons can be used to efficiently excite atoms that ultimately emit light. Alternatively energy absorbed by ions eventually gets transferred to the gas atoms as kinetic energy and simply heats up the gas.
As stated above, the positive column has approximately equal numbers of electrons and ions. Since the electrons have roughly 100 times the mobility of the ions, they will conduct 100 times more of the current than the ions in the positive column. Because most of the current flow in the positive column is in the electrons, virtually all of the power dissipated in the positive column goes into the kinetic energy of the electrons. This kinetic energy can be transferred to the excitation of atoms with efficiencies greater than 80% if the electric field is of the correct low value. Virtually all of the excited atoms generate ultraviolet photons which can further excite the phosphors to emit the desired visible light.
The negative glow has a high number of ions and a much smaller number of electrons. Even though the electrons have two orders of magnitude greater mobility than the ions, the ions are of such a large density that much of the power dissipated in the negative glow goes to the kinetic energy of the ions. However, the electric field in the negative glow is very high and therefore the electrons gain much higher kinetic energies than in the lower field of the positive column. The higher electron kinetic energies mean that the electrons can both excite and ionize the atoms. Electron energy used to ionize the atoms creates ions that flow to the cathode and are ultimately neutralized at the cathode surface.
While electron impact ionization of atoms is the source of ions and electrons that are needed to make the gas discharge conductive, it does not create any ultraviolet photons. Therefore the high electric fields in the negative glow allow a large amount of impact ionization which results in a much lower conversion efficiency of electron kinetic energy to ultraviolet photons. This UV conversion efficiency may be typically only 30% compared to 80% in the positive column.
It is known that the positive column exhibits an 80% total efficiency and that the negative glow exhibits an efficiency of 15%. This difference in efficiencies indicates why it is much more desirable to dissipate energy in the positive column than in the negative glow and is the fundamental reason that fluorescent lamps are designed to use the positive column and why they achieve a high luminous efficiency of 80 lumens per watt. To achieve this result, the fluorescent lamp design maximizes the power dissipation in the highly efficient positive column and minimizes it in the low efficiency negative glow.
One way that most fluorescent lamps reduce dissipation in the negative glow is to use a heated cathode that emits large numbers of electrons that serve to drive the gas discharge. This source of electrons reduces the voltage drop across the negative glow by an order of magnitude which, for equal currents, reduces the power dissipation in the negative glow by an order of magnitude. Such a reduction allows for more dissipation in the much more efficient positive column. The use of this same idea for PDPs would require a heated cathode for each of the hundreds of thousands of subpixels in the display. Because such an arrangement is impractical, it is difficult to reduce the power dissipation in the negative glow of a plasma display.
A second strategy for increasing the efficiency of the fluorescent lamp is to increase the length of the positive column. This is the reason that the common fluorescent lamp is a long tube. The positive column can be modeled as a resistor. Therefore the longer the positive column, the greater its resistance and the greater its power dissipation. The properties of the positive column allow it to be easily extended in length as long as there is sufficient voltage to establish the desired current across its resistance. This means that for a constant current, as the positive column is made longer, the voltage across the positive column needs to increase proportionally. Further, the longer the positive column, the more favorable is the ratio between the power dissipated in the positive column and that dissipated in the negative glow.
While the principle of making a gas discharge more efficient by using a long positive column is well known, it has not been successfully applied to PDPs. One reason is that it has long been thought that the long nature of the positive column is not practical for the very small subpixels of a plasma display and therefore, observers have stated that most of the light from a PDP comes from the negative glow.
FIG. 1 shows a prior art color AC PDP from U.S. Pat. No. 5,745,086. This structure utilizes ultraviolet light which is generated by a gas discharge to selectively excite red, green and blue phosphors to emit the desired full color visible light. FIGS. 2a-2c show typical cross sectional views of the subpixels in the AC PDP of FIG. 1. Such an AC PDP operates with AC voltages and provide write voltages which exceed the firing voltage of the ionizable gas at a given discharge site, as defined by selected column and row electrodes. The discharge is continuously "sustained" by applying an alternating sustain signal (which, by itself, is insufficient to initiate a discharge). The technique relies upon wall charges generated on the dielectric layers of the substrates which, in conjunction with the sustain signal, operate to maintain continuing discharges.
In order for an AC plasma panel to exhibit reliable operation, its wall charge states must be repeatable and standardized. More specifically, the wall charge states must exhibit repeatable values irrespective of a previous data storage state, so that succeeding address and sustain signals reliably cooperate to assure repeatable pixel site operation.
In FIGS. 1 and 2a-2c, PDP 10 includes a back substrate 12 upon which plural column address electrodes 14 are supported. Column address electrodes 14 are separated by barrier ribs 16 and are covered by red, green and blue phosphors 18, 20 and 22, respectively. A front transparent substrate 24 includes a pair of sustain electrodes 26 and 28 for each row of pixel sites. A dielectric layer 30 is emplaced on front substrate 24 and a magnesium oxide or similar high gamma material overcoat layer 32 covers the entire lower surface thereof, including all of sustain electrodes 26 and 28.
The structure of FIG. 1 is sometimes called a single substrate AC PDP since both sustain electrodes 26 and 28, for each row, are on a single substrate of the panel. An inert gas mixture is positioned between substrates 12 and 24 and is excited to a discharge state by a sustain signal applied by sustain electrodes 26 and 28. The discharging inert gas produces ultra-violet light that excites the red, green and blue phosphor layers 18, 20 and 22, respectively to emit visible light. If the driving voltages applied to column address electrodes 14 and sustain electrodes 26, 28 are appropriately controlled, a full color image is visible through front substrate 24.
The table shown in FIG. 2d provides typical dimensions of prior art PDPs (in micrometers) for various designs. Designs F, N, M and P are designs used in practical displays by various manufacturers. Note that for these designs, the gap distance between the front substrate sustain electrodes, called the sustain gap (SusG) is usually approximately equal to the gap distance between the front substrate and the rear substrate, referred to as the substrate gap (SubG). This is illustrated by the ratio SusG/SubG which ranges between 0.84 and 1.23 for the four prior art designs.
While many different dimensions have been successfully used, the approximate equality between these two gaps has been maintained. Note also that the sustain gap is always less than the distance between the sustain electrode of one subpixel and the sustain electrode of a neighboring subpixel which is referred to as the inter pixel gap (IPG). This is illustrated by the ratio SusG/IPG which ranges between 0.29 and 0.37 for the four prior art designs.
If the IPG is not considerably larger than the SusG, there will be strong interaction between subpixels that will cause operational failures. More specifically, if the IPG is smaller than the SusG, then when the sustain signal is applied, the electric field across the IPG will be larger than the electric field across the SusG. This will allow a discharge to occur along the IPG which would modify the charge on the sustain dielectric layers and substantially modify the operation of the discharge along the sustain gap.
It is therefore an object of this invention to provide a full color PDP which exhibits improved image brightness and luminous efficiency when compared to prior art PDPs.
It is a further object of this invention to provide a full color PDP wherein subpixel sites utilize positive column discharges to achieve improved luminous efficiencies and high levels of light emission.